High-speed imaging devices (high-speed video cameras) for taking consecutive images of high-speed phenomena such as explosions, destructions, combustions, collisions and discharges for only a short period of time have been conventionally developed (for example, refer to Non-Patent Document 1 and other documents). Such high-speed imaging devices need to perform an ultrahigh-speed imaging operation that exceeds a level of approximately one million frames per second. Accordingly, they use solid-state image sensors capable of high-speed operations, which have special structures different from those of the imaging devices conventionally used in normal video cameras, digital cameras and similar devices.
One example of this type of solid-state image sensor is disclosed in Patent Document 1 and other documents, which is referred to as an in-situ storage image sensor (IS-CCD). An outline of this image sensor is as follows: A storage CCD, which also serves for the transfer of a specified number of recorded images (frames), is provided for each photodiode as a photo-receiver. During an imaging operation, pixel signals resulting from photoelectric conversion by the photodiode are sequentially transferred to the storage CCD. After the imaging operation is completed, the pixel signals corresponding to the specified number of record frames stored in the storage CCD are collectively read, and the images corresponding to the specified number of record frames are reproduced outside the image sensor. During the imaging operation, pixel signals exceeding the specified number of image frames are discarded from the oldest ones. Thus, the latest set of pixel signals corresponding to the specified number of frames are held in the storage CCD. This means that, when the transfer of pixel signals to the storage CCD is suspended at the completion of the imaging operation, one can obtain the latest series of images ranging from the completion of the imaging operation back through a period of time corresponding to the specified number of record frames.
In such a high-speed imaging operation, the period of time in which the photodiodes are exposed to light to obtain one frame of pixel signals is extremely short as compared to the normal imaging operation. For example, in the case of a high-speed imaging operation at one million frames per second, the exposure time for one frame of image is equal to or shorter than 1 μsec. Therefore, to ensure an adequate detection sensitivity, it is necessary to receive the largest possible amount of light by the photodiode in each pixel. Accordingly, it is desirable to provide the photodiode with the largest possible light-receiving surface. However, increasing the size of the light-receiving surface of the photodiode causes the following problem.
FIG. 21 is a plan view showing a pixel structure using a normal type of embedded photodiode. This structure has a floating diffusion FD located on one side of the photodiode PD. When a transfer transistor TX provided between them is turned on, photocharges flow from the photodiode PD to the floating diffusion FD and are stored therein. RG is a transistor for resetting the floating diffusion FD.
In this structure, when the size of the light-receiving surface of the photodiode is increased, the photocharges produced by the photodiode PD requires a non-negligible length of time to reach the floating diffusion PD, and a portion of the charges cannot be able to reach the floating diffusion within the predetermined short photoelectric conversion time. As a result, the photocharge usage efficiency will deteriorate, and the detection sensitivity will not be improved to the expected level even if the size of the light-receiving surface of the photodiode; rather, it will be a major cause of image-quality deterioration.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-345441    Non-Patent Document 1: Kondo et al., “Kousokudo Bideo Kamera HyperVision HPV-1 no Kaihatsu (Development of “HyperVision HPV-1” High-Speed Video Camera)”, Shimadzu Hyouron (Shimadzu Review), Shimadzu Hyouron Henshuu-bu, Sep. 30, 2005, Vol. 62, No. 1/2, pp. 79-86